1. Field of the Invention
This invention relates generally to the heating of equipment used to process plastic material. More particularly, the invention relates to induction heating a metal barrel of the type used for injection molding of plastics.
2. Description of the Prior Art
Solid plastic feed material enters the feed end of a barrel and then is sheared, mixed and metered by a rotating screw, which forces the material in a molten state through a nozzle at the discharge end. To help melt the plastic, band-heaters, arranged on the barrel's outer surface, are heated from an electric power source.
The electrical circuitry of the band-heaters is usually arranged so that the barrel can be heated in multiple controllable zones along its length, with one thermocouple located in the barrel wall per zone to provide temperature measurement feedback.
As the unheated plastic feed material enters the barrel, the temperature of the barrel wall drops in the vicinity of the feed material inlet, resulting in a demand for heat in that zone.
Because of the relatively slow thermal response of band-heaters, conventional barrel temperature measurement and control hardware and software is not designed to detect or respond to the drop in barrel temperature that occurs each machine cycle. Cycle times typically range from 5 to 150 seconds and typically increase with part weight. Consequently, a conventional barrel temperature control using band-heaters is incapable of responding to any temperature variation having a period of a few minutes or less, whether the variation derives from the feed material's cyclical deposition in the barrel, or from more unpredictable factors such as cooling capacities, ventilation changes around the machine, changes in feed material properties such as moisture content, changes in mold design and capacity, and/or the machine setup. Because of this, on conventional band-heated applications, thermocouple depth within the barrel wall is often intentionally shallow, so that uncontrollable short-term temperature variations will go undetected.
AC induction has also been used to heat injection molding and extrusion barrels, by inducing eddy currents within the barrel wall to produce direct resistive heating of the barrel. By comparison, induction is typically capable of heating the barrel as fast as the introduction of cold feed material is able to cool it, so deeper thermocouple placement combined with induction heating will allow these temperature variations to be detected and controlled.
Simple control software can be configured so that induction heat is applied in a synchronized fashion, once per machine cycle as the cold feed material is introduced and the process temperature drops sufficiently below the target value to trigger a control output. However, induction heating systems added to existing injection molding machines must work with the machine's existing temperature control software, which typically provides duty-cycle outputs that prevent synchronized control.
Conventional temperature control software, whether resident in a PLC, PC or other microcontroller platform, usually provides one duty-cycled output per control zone that pulses power to the zone's band-heaters through a contact closure circuit, using relays or contactors. The power output interval used in a duty-cycle scheme is typically configurable and fixed, and is chosen to be a small fraction of the band-heaters' time-constant, typically less than 30 seconds, versus several minutes, and ideally, also is much shorter than the machine's cycle time.
An asynchronous control in which the control output interval is not equal to the machine cycle, uses the band-heaters' large thermal inertia to simulate a smooth constant power output over time, thereby permitting the use of inexpensive discrete controller outputs in lieu of expensive analog power controls. However, asynchronous control unfortunately prevents conventional temperature control software from taking advantage of induction's much faster response characteristics to add heat to the process synchronously with the need, i.e., or once per machine cycle. Instead, it forces control to be non-synchronous with the process, thereby increasing process temperature variability.